Having previously obtained indirect evidence from frogs that olfactory discrimination may depend upon the space-time pattern by which the molecules of different odorants are distributed across the olfactory mucosa, we will proceed as follows: By presenting tritium-labeled odorants we will map exactly how the molecules of different odorants are distributed across the length and breadth of the receptor sheet. We will also follow their distribution vertically through the depth of the mucosa, noting particularly any differential filtering of different chemicals by either the mucus or the olfactory cilia. The radioactive odorant molecules will also be followed to determine the pathways by which odorants are removed from the mucosa. By electrophysiologically recording from single mitral cells in the olfactory bulb which had previously been shown by electrical stimulation to receive their inputs from particular regions of the olfactory receptor sheet, we will determine whether the differential molecular distributions established across the mucosa are topographically represented by a group of mitral cells in the bulb. Using chemicals with known mucosal distribution patterns and by recording from single mitral cells with known regional input, we will determine whether particular regions of the olfactory mucosa are mutually inhibitory, facilitory, or without effect. BIBLIOGRAPHIC REFERENCES: Hornung, D.E. and Mozell, M.M. Factors influencing the differential sorption of oderant molecules across the olfactory mucosa. J. Gen. Physiol., 1977, in press. Mozell, M.M. Processing of olfactory stimuli at peripheral levels. Chemical Signals in Vertebrates (D. Muller-Schwarze and M.M. Mozell, eds.), Plenum Press, N.Y., 1977, in press.